Although some edible oils are used per se, by far the largest portion are hydrogenated, or hardened, prior to their end use. The purpose of such hydrogenation is to increase the stability of the final product. For example, processed soybean oil is susceptible to oxidation resulting in deterioration of its organoleptic properties upon storage even at ambient temperature. Where the oil is to be used at higher temperatures, for example, as a frying oil, the adverse organoleptic consequences of oxidation become even more pronounced.
The commonly accepted origin of oxidative deterioration is attributed to highly unsaturated components, such as the triene moiety, linolenate, in soybean oil. Partial hydrogenation to remove most of this component leads to a marked increase in the oxidative stability of the resulting product, thereby facilitating storage and permitting unobjectionable use at higher temperatures. Ideally, one desires this hydrogenation to be highly specific, reducing only triene to the diene, linoleate, without effecting cis to trans isomerization. In practice, this goal is unachievable.
The edible fats and oils which are the subject of this invention, collectively referred to as fatty materials, are triglycerides of fatty acids, some of which are saturated and some of which are unsaturated. In vegetable oils, the major saturated fatty acids are lauric (12:0), myristic (14:0), palmitic (16:0), stearic (18:0), arachidic (20:0), and behenic (22:0) acids. The notation, "18:0,"for example, means an unbranched fatty acid containing 18 carbon atoms and 0 double bonds. The major unsaturated fatty acids of vegetable oils may be classified as monounsaturated, chief of which are oleic (18:1) and erucic (22:1) acids, and polyunsaturated, chief of which are the diene, linoleic acid (18:2), and the triene, linolenic acid (18:3). Unhardened vegetable fats and oils contain virtually exclusively cis-unsaturated acids.
In the context of partial hydrogenation, the ultimate goal is the reduction of triene to diene without attendant transacid formation or saturate formation. In practice, it is observed that partial reduction results in lowering both triene and diene and increasing the monoene, saturate, and trans level. Because it is desired that the product of partial hydrogenation itself be a liquid oil relatively free of sediment or even cloudiness upon storage at, for example, 10.degree. C., the formation of saturated and trans acids in such hydrogenation is a vexing problem. Removal of these solids, whose relative amount is measured by the Solid Fat Index (SFI), is a relatively costly and inefficient process attended by large losses associated with the separation of gelatinous solids from a viscous liquid. It is known in the art that such solids are composed largely of triglycerides containing at least one saturated fatty acid moiety and/or trans monounsaturated fatty acid moiety with the predominant culprits having at least 18 carbon atoms. It is further known in the art that fatty acid analysis alone may be an insensitive analytical tool, that is to say, two products of hydrogenation of, for example, soybean oil may be vastly different in their SFI while having virtually identical fatty acid analysis. This arises because the distribution of the saturated moieties in the triglyceride is important. The solubility in the soybean oil of disaturated triglycerides is much less than twice the amount of monosaturated triglycerides, and the solubility of monosaturated triglycerides may depend upon whether the other fatty acid moieties of the triglyceride are monounsaturated, diunsaturated, etc., and may also depend upon whether the saturated portion is at the one- or two-position of the triglyceride. Hence, hydrogenation of edible fats and oils is largely an empirical process, whose analytical tools include SFI supported by fatty acid analysis.
Although catalyst supports generally have been viewed as passive agents for carrying catalysts, such as zerovalent metals, in a highly dispersed state, there recently has developed a body of knowledge, both theoretical and experimental, showing strong metal-support interaction (SMSI) in a class of metal oxides bearing zerovalent metals. S. J. Tauster, S. C. Fung, R. T. K. Baker, and J. A. Horsley, Science, 211, 1121 (1981). Catalysts exhibiting such SMSI have been prepared in U.S. Pat. No. 4,149,998. The expectation that SMSI would significantly alter the catalytic properties of a dispersed metal, even though the alteration may be unpredictable, has also been experimentally confirmed, largely in the case of Fischer-Tropsch catalysts as shown in U.S. Pat. Nos. 4,206,134 and 4,206,135.
I have found that zerovalent platinum and palladium dispersed on supports manifesting SMSI show surprisingly increased selectivity in the reduction of edible oils and fats without appreciable alteration of their activity. This observations forms the basis for the invention described herein, which is a method for selectively hydrogenating fatty materials by either a batch or continuous process. The utility and importance of this invention is readily discerned when it is appreciated that commercial methods of continuous reductions of fatty materials are at once highly desirable and extraordinarily difficult, with no general method presently available for widespread industrial usage.